If an engine rotates at a velocity over the upper tolerance limit, the durability of the engine falls. This is an undesirable result. In order to prevent such a problem technology described in, for example, Japanese Kokai Patent Application No. 2004-245191 has been proposed. According to this technology, when over-rotation of the engine occurs due to ON/OFF control of the fuel supply, the catalyst of the engine exhaust system is adversely affected. This leads to deterioration in the operability of the engine. In consideration of this mechanism, over-rotation of the engine is prevented by reducing the opening degree of the throttle valve that controls the intake air flow rate of the engine.